1. Field of the Invention
The present invention relates to the management of electric power to be supplied to a disk storage device, and more particularly to a method of reducing power dissipation.
2. Background Art
Making the power dissipation of electronic equipment such as personal computers (PCs) even smaller is important to further environmental protection efforts. Particularly, for portable equipment such as notebook type PCs, even a further saving of power has been demanded in order to make long use possible by an incorporated power source.
The hard-disk drive (HDD), employed as a storage device for these pieces of equipment, has an idle mode in which a track following operation is performed but no read or write operations are performed, in addition to an active mode in which no operation is performed for power saving, in order to reduce power dissipation. To further reduce power dissipation, the HDD also has a plurality of power saving modes, such as a standby mode in which a spindle motor for spinning the disk stands by.
To reduce power dissipation on average, it is effective to make a transition to a power saving mode of as low a level (less power dissipation) as possible during non-operation in which there is no command from a host computer.
On the other hand, to prevent the time to respond to a command from the host computer from becoming long, there is a need to return from the power saving mode to the active mode as soon as the command is received.
In general, more time and power become necessary when making a transition to a power saving mode of a lower level or when returning to the active mode.
Therefore, in the case where the interval to receive a command is short and the non-operating time is short, a determination should be made whether the power that will be dissipated by making a transition to the power saving mode will be greater than the power that can be saved by entering that power saving mode and as a result will actually the average power dissipation of the HDD. In addition, the response to the command will be delayed by a time to return to the active mode.
For such reasons, there is a need to optimize the timing for entering the power saving mode. The following methods have been considered for that purpose.
(1) Control Method by a Simple Timer:
In this method, when more than a constant time has elapsed since a command was executed just before, it is judged that the next command will not come for a while, and a transition to the power saving mode is made. The xe2x80x9cconstant timexe2x80x9d here means times respectively corresponding to a fixed value and a value specified by a command from the host computer.
However, in this method, when the interval between commands is slightly longer than the above-mentioned constant time, for example, the next command comes immediately after the power saving mode. For this reason, this method returns to the active mode, and consequently, there are cases where average power dissipation is increased.
(2) Adaptive Battery Life Extender (ABLE) Method:
In this method, at intervals of the past constant time, the number (density) of commands that the HDD received within that time is checked. In the case where the density of the commands received within the constant time is high, the time to make a transition to the power saving mode is shortened. In the case where the density of the commands is low (the interval between the commands is long), the time to make a transition to the power saving mode is lengthened.
With this method, in the case where the HDD receives commands at constant intervals, the possibility of the next command coming immediately after an entry of the power saving mode can be reduced. However, in the case where the interval between commands changes at random, a transition to the power saving mode cannot be always made at optimum timing. Also, since the timing to make a transition to the power saving mode is determined based on the density of commands with the time, at which a command was received, as a reference, the apparent density will be reduced in the case where the time for the HDD to process a command is long, and consequently, there is a possibility that a transition to the power saving mode will be made at improper timing.
(3) Enhanced Adaptive Battery Life Extender (ABLE2) Method
This method is an expansion of the ABLE method in subsection (2). That is, the power saving mode to be controlled by the ABLE method is only one, but this number is increased to three. Furthermore, this method also makes it possible to automatically make a transition to a power saving mode of a lower level (e.g., in an HDD equipped with a loading/unloading mode, a mode of unloading the read/write head).
The timing to make a transition to another power saving mode is identical in this method identical and the ABLE method. However, ABLE2 differs from the ABLE method in that ABLE2 defines three kinds of functions, depending upon a pattern of command intervals (i.e., a case of receiving commands at constant intervals, a case that does not receive a command for a while after the first command has been received, and a case of receiving commands at random intervals) and thereby makes a transition from an active mode to a power saving mode, or from a power saving mode to another mode, at timing as good as possible.
Problems with these prior art systems include that they do not calculate the electric power dissipation that is expected to be saved in a transition to the power saving mode. Therefore, in conventional systems, the additional electric power that is dissipated for a transition to a power saving mode or a return from the power saving mode and the electric power dissipated for the delayed time of command response can be greater than the electric power dissipation to be saved during the transition to the power saving mode, depending upon the command interval and the timing to make a transition to the power saving mode. Consequently, there are cases where the object of saving power is not achieved.
Also, the density of commands is calculated with the time at which a command was received as a reference. Therefore, in the case where an HDD takes time to process a command, it is late in providing the command completion signal and accordingly the host computer issues the next command late. For this reason, although the non-operating time of the HDD is short (i.e., although the rate of operation is high), the apparent density of commands that are received from the host computer is reduced, and consequently, there is a possibility that a transition to the power saving mode will be made at improper times.
It is an object of the present invention to further reduce the power dissipation of a HDD.
Another object of the invention is to provide a method and a system that are capable of computing the electric power dissipation that is expected to be saved when a transition to the power saving mode is made.
Still another object of the invention is to provide a method and a system that calculate the density of commands that a HDD receives from a host computer truly in accordance with the non-operating time of the HDD, while overcoming the above-mentioned disadvantage that, although the non-operating time of the HDD is short, the apparent density of commands to be received from the host computer is reduced and therefore a transition to the power saving mode is made at improper timing.
According to the present invention, the electric power dissipation additionally required for a transition to a power saving mode and a return from the power saving mode and the electric power dissipation equivalent to the amount of a response delay time with respect to a command are previously compared with the electric power dissipation that is expected to be saved during a transition to the power saving mode.
Also, according to the present invention, it is supposed that the next command comes with the same time distribution as the time distribution of past command intervals to the HDD. The electric power dissipation, which is expected to be saved when the HDD makes a transition to the power saving mode, is calculated at a plurality of times.
In addition, according to the present invention, an average delay time for returning from the power saving mode in order to execute the next command is calculated along with the electric power dissipation that is expected to be saved.
Furthermore, according to the present invention, within a range where the above-mentioned calculated average delay time is shorter than a time specified by a host computer, a transition to the power saving mode is made at times when the amount of electric power dissipation that can be saved is greatest.
Moreover, according to the present invention, the past command interval employed in calculations employs the time between completion of the execution of the previous command and reception of the next command, not the time between reception of the previous command and reception of the next command. Namely, the time when interface is not operated (idle state) is regarded as command interval.
Finally, according to the present invention, the influence of the past command interval that is employed in calculations is reduced by half at intervals of a constant time so that the latest access pattern is reflected most significantly in the calculations.